In many WDM components of optical communication systems, such as optical power (channel) monitors or dynamic gain (channel) equalizers, the incoming multi-wavelength signal is first spectrally dispersed, then detected and/or processed on a per wavelength basis, and, optionally, multiplexed back into the multi-wavelength outgoing signal. A common method of achieving the required functionality typically relies on hybrid integration of discrete passive devices, such as optical spectral analyzer, and active devices, such as photodetectors,; attenuators, or amplifiers. Examples of this approach are found in U.S. Pat. No. 6,327,075 by Ishii, issued 4 Dec. 2001 and U.S. Pat. No. 6,268,945 by Roberts, issued 31 Jul. 2001. While simple engineering solutions resulting in hybrid components are functionally attractive for some applications, they may be prohibitively cumbersome and costly for others. The search for more compact and cost efficient solutions has naturally resulted the development of integrated planar waveguide components, e.g. those reported by C. Cremer et al, in “Grating Spectrograph Integrated with Photodiode Array in InGaAsP/InGaAs/InP”, IEEE Photon. Technol. Lett, Vol. 4, P. 108, 1992; by J. B. Soole et al, in “Integrated Grating Demultiplexer and pin array for High-Density Wavelength Division Multiplexed Detection at 1.55 μm”, Electron. Lett., Vol. 29, P. 558, 1993; by M. R. Amersfoort et al, in “Low-Loss Phased Array Based 4-channel Wavelength Demultiplexer Integrated with Photodetectors”, IEEE Photon. Technol. Lett, Vol. 6, P. 162, 1994; by M. Zirngibl et al, in “WDM Receiver by Monolithic Integration of an Optical Preamplifier, Waveguide Grating Router and Photodetector Array”, Electron. Lett., Vol. 31, P. 581, 1995; by C. R. Doerr et al, in “Dynamic Wavelength Equalizer in Silica Using the Single-Filtered-Arm Interferometer”, IEEE Photon. Technol. Lett., Vol. 11, P. 581, 1999; by P. M. J. Schiffer et al, in “Smart Dynamic Wavelength Equalizer with On-Chip Spectrum Analyzer”, IEEE Photon. Technol. Lett., Vol. 12, P. 1019, 2000. In these components, the optical spectral analyzer most commonly used is either an echelle waveguide grating or an arrayed waveguide grating (AWG) and the active devices are integrated within the passive ridge waveguides, physically separating the individual wavelength channels. As a result, a compact and inexpensive integrated component for use in WDM systems is produced, provided a method for monolithic integration of active and passive waveguides is found that is feasible given existing production techniques as well as being cost efficient.
It is not at all trivial to combine passive waveguides used in optical spectral analyzers with active waveguide devices, such as WPDs, EAAs or SOAs, within the same semiconductor structure. This is because the passive and active semiconductor optical components typically have different bandgaps relative to their operating photon energy. One having skill in the art of designing active waveguide devices will be aware that the operating photon energy should be above the bandgap in a photodetector, close to the bandgap in an amplifier and well below the bandgap in a passive waveguide. Various methods for monolithic integration of active and passive semiconductor waveguides, which resolve this fundamental problem have been proposed, most of them involving one or both of the following major techniques: butt-coupling and evanescent-field coupling, as described in a review paper by R. J. Deri, “Monolithic Integration of Optical Waveguide Circuitry with III–V Photodetectors for Advanced Lightwave Receivers”, IEEE J. Lightwave Technol., Vol. 11, P. 1296, 1993. The former is straightforward but expensive and unreliable due to its difficulty in implementation, since it requires complex epitaxial growth techniques such as etch and re-growth, e.g. reported by S. Lourdudoss et al, in “Uniqueness of Hydride Vapour Phase Epitaxy in Optoelectronic Device Fabrocation”, Int. Conf. Indium Phosphide and Related Materials, May 11–15 May 1998, Tsukuba, Japan, P. 785, or selective area growth, e.g. reported by D. Jahan et al, “Photonic Integration Technology without Semiconductor Etching” Int. Conf. Indium Phosphide and Related Materials, 16–20 May 1999, Davos, Switzerland, P. 28. The latter uses simple single-step epitaxial growth, but has problems with coupling efficiency between passive and active waveguides, when the active waveguide is grown on top of the passive one. In attempts to achieve good and wavelength-independent coupling efficiency for two vertically integrated waveguides, various sophisticated techniques have been proposed, e.g. such as those disclosed by B. Mersali et al, in “Optical-Mode Transformer: A III–V Circuit Integration Enabler”, IEEE J. Selected Topics in Quantum Electron., Vol. 3, P. 1321, 1997; by P. V. Studenkov et al, “Efficient Coupling in Integrated Twin-Waveguide Lasers Using Waveguide Tapers”, IEEE Photon. Technol. Lett., Vol. 11, P. 1096, 1999; or by S. S. Saini et al, “Passive-Active Resonant Coupler (PARC) Platform with Mode Expander”, IEEE Photon. Technol. Lett., Vol. 12, P. 1025, 2000. However, none of them are both simple to implement and cost efficient at present and hence these approaches also do not solve the problem of developing reliable and inexpensive integrated devices for sale and distribution in the very near term.